Correct UVs Blender 2.8
You should loop cut (Ctrl+R) or edge slide (G+G) first

And then, there'll be some options. Then, check the Correct UVs


After the modal tool is run the Loop Cut and Slide Options. These options are available in the Adjust Last Operation panel.

https://docs.blender.org/manual/en/latest/modeling/meshes/tools/loop.html
https://docs.blender.org/manual/en/latest/modeling/meshes/editing/edge/edge_slide.html